vhost: fix maximum of queues

vhost: fix maximum of queues

Currently, the maximum value of rx/tx queues are kept by EAL. But,
the value is used like below with different meanings in vhost PMD.
 - The maximum value of current enabled queues.
 - The maximum value of current supported queues.

This wrong double meaning will cause an issue like below steps.

* Invoke application with below option.
  --vdev 'eth_vhost0,iface=<socket path>,queues=4'
* Configure queues like below.
  rte_eth_dev_configure(portid, 2, 2, ...);
* Configure queues again like below.
  rte_eth_dev_configure(portid, 4, 4, ...);

The second rte_eth_dev_configure() will fail because both
the maximum value of current enabled queues and supported queues
will be '2' after calling first rte_eth_dev_configure().

To fix the issue, the patch adds another variable to keep the maximum
number of supported queues in vhost PMD.

Fixes: 23981fb0d78b ("vhost: Add vhost PMD")

Signed-off-by: Tetsuya Mukawa <mukawa@igel.co.jp>
Acked-by: Ciara Loftus <ciara.loftus@intel.com>